Absorbent embryonic webs are a staple of everyday life. Absorbent embryonic webs include cellulosic fibrous structures, absorbent foams, etc. Cellulosic fibrous structures have become a staple of everyday life. Cellulosic fibrous structures are found in facial tissue, toilet tissue and paper toweling.
In the manufacture of cellulosic fibrous structures, a wet embryonic web of cellulosic fibers dispersed in a liquid carrier is deposited onto a forming wire. The wet embryonic web may be dried by any one of or combinations of several known means. Each of these known drying means will affect the properties of the resulting cellulosic fibrous structure. For example, the drying means and process of drying can influence the softness, caliper, tensile strength, and absorbency of the resulting cellulosic fibrous structure. Importantly, the means and process used to dry the cellulosic fibrous structure also affects the rate at which it can be manufactured, without being rate limited by such drying means and process.
An example of one drying means is felt belts. Felt drying belts have long been used to dewater an embryonic cellulosic fibrous structure through capillary flow of the liquid carrier into a permeable felt medium held in contact with the embryonic web. However, dewatering a cellulosic fibrous structure with a felt belt results in overall uniform compression and compaction of the embryonic cellulosic fibrous structure web to be dried.
Felt belt drying may be assisted by a vacuum, or may be assisted by opposed press rolls. The press rolls maximize the mechanical compression of the felt against the cellulosic fibrous structure. Examples of felt belt drying are illustrated in U.S. Pat. No. 4,329,201 issued May 11, 1982 to Bolton and U.S. Pat. No. 4,888,096 issued Dec. 19, 1989 to Cowan et al.
Drying a cellulosic fibrous structure via capillary flow, using a porous cylinder having preferential pore sizes is known in the art as well. Examples of such capillary flow drying techniques are illustrated in commonly assigned U.S. Pat. No. 4,556,450 issued Dec. 3, 1985 to Chuang et al., incorporated herein by reference, U.S. Pat. No. 5,598,643, issued Feb. 4, 1997 in the names of Chuang et al., and U.S. Pat. No. 4,973,385 issued Nov. 27, 1990 to Jean et al.
Drying cellulosic fibrous structures through vacuum dewatering, without the aid of felt belts is known in the art. Vacuum dewatering of the cellulosic fibrous structure mechanically removes moisture from the cellulosic fibrous structure using vacuum shoes and vacuum boxes. The vacuum deflects discrete regions of the cellulosic fibrous structure into the drying belt. Preferably the drying belt is a through air drying belt having a resinous patterned framework with deflection conduits therethrough, as disclosed in commonly assigned U.S. Pat. No. 4,637,859 issued to Trokhan and incorporated herein by reference. Vacuum dewatering on such a belt produces a multi-region cellulosic fibrous structure having a high density essentially continuous network and discrete low density regions distributed therein.
Dewatering with such a belt yields a cellulosic fibrous structure having different amounts of moisture in the two aforementioned regions. The different amounts of moisture in the different regions of the cellulosic fibrous structure can rate limit the papermaking process. Such limitation occurs because the two regions will dry at different rates. The region having the slower drying rate will then control the overall rate of the papermaking process.
In yet another drying process, considerable success has been achieved by through-air drying the embryonic web of a cellulosic fibrous structure. In a typical through-air drying process, a foraminous air permeable belt supports the embryonic web to be dried. Air flow passes through the cellulosic fibrous structure and through the permeable belt. The air flow principally dries the embryonic web by evaporation. Regions coincident with and deflected into the foramina of the air permeable belt are preferentially dried and the caliper of the resulting cellulosic fibrous structure is increased. Regions coincident the knuckles in the air permeable belt are dried to a lesser extent.
Several modifications and improvements to the air permeable belts used for through-air drying have been accomplished in the art. For example, the air permeable belt may be made with a relatively high open area. Or, the belt may be made to have reduced air permeability. Reduced air permeability may be accomplished by applying a resinous mixture to obturate the interstices between woven yarns in the belt. The drying belt may be impregnated with metallic particles to increase its thermal conductivity and reduce its emissivity. Preferably, the drying belt is constructed from a photosensitive resin comprising a continuous network. The drying belt may be specially adapted for high temperature airflows. Examples of such through-air drying technology are found in U.S. Pat. No. Re. 28,459 reissued Jul. 1, 1975 to Cole et al.; U.S. Pat. No. 4,172,910 issued Oct. 30, 1979 to Rotar; U.S. Pat. No. 4,251,928 issued Feb. 24, 1981 to Rotar et al.; commonly assigned U.S. Pat. No. 4,528,239 issued Jul. 9, 1985 to Trokhan; and U.S. Pat. No. 4,921,750 issued May 1, 1990 to Todd.
Additionally, several attempts have been made in the art to regulate the drying profile of the cellulosic fibrous structure while it is still an embryonic web to be dried. Such attempts may use either the drying belt, or an infrared dryer in combination with a Yankee hood. Examples of profiled drying are illustrated in U.S. Pat. No. 4,583,302 issued Apr. 22, 1986 to Smith and U.S. Pat. No. 4,942,675 issued Jul. 24, 1990 to Sundovist.
The foregoing art, even that specifically addressed to through-air drying, does not address the problems encountered when drying a multi-region cellulosic fibrous structure. As noted above, different regions of through air dried paper have different moisture contents. But a first region of the cellulosic fibrous structure, having a lesser density or basis weight than a second region, will typically have relatively greater airflow therethrough than the second region will have. This relatively greater airflow occurs because the first region of lesser density or basis weight presents proportionately less flow resistance to the air passing through the embryonic web than the second region. Such differential air flow does not offset, and may even increase, the differential moisture contents of the different regions.
This problem is exacerbated when the multi-region cellulosic fibrous structure to be dried is transferred to a Yankee drying drum. On a Yankee drying drum, only certain regions of the cellulosic fibrous structure contact the circumference of a heated cylinder. Typically the most intimate contact with the Yankee drying drum occurs at the high density or high basis weight regions. These regions have more moisture than the low density or low basis weight regions.
Hot air from a hood may be introduced to the surface of the cellulosic fibrous structure opposite the heated cylinder. Preferential drying of this surface of the cellulosic fibrous structure occurs by convective transfer of the heat from the airflow in the Yankee drying drum hood. To allow complete drying of the high density and high basis weight regions of the cellulosic fibrous structure to occur and to prevent scorching or burning of the already dried low density or low basis weight regions by the air from the hood, the Yankee hood air temperature must be decreased and/or the residence time of the cellulosic fibrous structure in the Yankee hood must be increased, slowing the production rate. Accordingly, the production rate of the cellulosic fibrous structure must be slowed, to compensate for the greater moisture in the high density or high basis weight region.
One improvement in the art which addresses this problem is illustrated by commonly assigned U.S. Pat. No. 5,274,930 issued Jan. 4, 1994 to Ensign et al. and disclosing limiting orifice drying of cellulosic fibrous structures in conjunction with through-air drying, which patent is incorporated herein by reference. This patent teaches an apparatus utilizing a micropore drying medium which has a greater flow resistance than the interstices between the fibers of each region of the cellulosic fibrous structure. The micropore medium is the limiting orifice in the through-air drying process, so that a more uniform moisture distribution is achieved in the drying process.
Yet a further improvement to the apparatus disclosed in Ensign et al. '930 is the apparatus disclosed in commonly assigned U.S. Pat. No. 5,581,906 issued Dec. 10, 1996 to Ensign at al. and incorporated herein by reference. Ensign et al. '906 discloses a micropore drying apparatus having multiple zones and which more efficiently dries the cellulosic fibrous structure than the types of apparatus disclosed in the prior art.
The foregoing micropore drying apparatuses should desirably provide a medium which both limits the air flow through the cellulosic fibrous structure and has sufficient bending fatigue strength to withstand the cyclic loading inherent to papermaking with the claimed apparatus. For example, the medium may be executed as the covering of an axially rotatable roll. As the roll and medium are rotated, any portion of the medium alternately receives both positive and negative pressure loads. Reversing the loading from positive to negative cycles the medium with an alternating stress that must be withstood by the medium. Thus, the medium must have adequate bending fatigue strength, to withstand this cyclic loading.
One solution to the problem of providing adequate bending fatigue strength might be to simply to make the medium stronger. However this solution, without more, brings other problems. As the medium becomes stronger, it typically becomes thicker and may have less open area. A medium having less open area encounters a greater pressure drop than a medium having relatively more open area. The benefits of minimizing pressure drop are known and discussed in the aforementioned Ensign et al. '906 patent. Furthermore, as the medium becomes thicker, it also becomes more difficult to fabricate.
Accordingly, it is an object of this invention to provide a medium for use with a micropore apparatus particularly the apparatus of the aforementioned Ensign et al '906 and the Ensign et al. '930 patents. It is also an object of the present invention to provide a medium usable with the capillary dewatering apparatus, such as the apparatuses of the aforementioned Chuang et al. '450 patent or the aforementioned Chuang et al. '305 application. It is also an object of the present invention to provide a medium usable with conventional felt dewatering and through air drying.
It is further an object of this invention to provide such a medium which provides both adequate bending fatigue strength and a relatively small pressure drop. Particularly, it is an object to provide such a medium that has a relatively small pressure drop.